add MSC (Mass Storage Class) support
Allow more than one usbd_register_set_config_callback

In a composite device if one want to separate code
for each interface, usbd_register_set_config_callback
can now register more than one callback.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
USB: Fix definition of function according to previous commit.
Merge remote-tracking branch 'upstream/master'